In a known optical system incorporated in a signal pickup device of an optical video and/or audio disc player, a beam of light emitted from a light source such as a semiconductor laser radiation element is directed to a beam splitter prism directly or through a lens system. The beam splitter prism has a parallelepiped construction and is composed of a pair of triangular or equilaterally pentahedral component segments having their respective base surfaces cemented together. The base surfaces of the component segments are inclined at the angles of 45 degrees to the side surfaces of the segments and constitute in combination a light polarization plane or a semi-reflective mirror plane by preferance. The beam of light admitted into the beam splitter prism is thus partially transmitted through the light polarization plane or the semi-reflective mirror plane of the prism and is directed to an information-carrying surface of a rotating video and/or audio disc directly or through a lens system. If the beam splitter prism is of the type having a light polarization plane formed therein, a quarter-wave plate is provided between the beam splitter prism and the video and/or audio disc so that a beam of linearly polarized light emanating from the beam splitter prism is circularly polarized into two component segments respectively oscillating on planes inclined at 45 degrees to the oscillating plane of the beam of light incident on the polarization plane.
The information-carrying surface of the rotating video and/or audio disc is scanned by the beam of light incident thereon so that a beam of light carrying the information read out from the information-carrying surface of the disc is reflected therefrom. The reflected beam of light is directed backwardly to the beam splitter prism and is re-directed from the light polarization plane or the semi-reflective mirror plane of the prism toward a light-sensitive transducer element such as a photo diode usually through a focussing lens provided between the prism and the transducer element. The signals carried by the beam of light incident on the transducer element is thus converted into electric signals representative of the information read out from the video and/or audio disc. In the optical system using the beam splitter prism of the type having the light polarization plane, the component beams of circularly polarized light reflected from the video and/or audio disc are united by the quarter-wave plate into a beam of linearly polarized light. The beam of linearly polarized light thus emanating backwardly from the quarter wave plate oscillates in a plane perpendicular to the transmission axis of the light polarization plane of the beam splitter prism and is totally reflected from the light polarization plane toward the above mentioned light-sensitive transducer element.
A drawback inherent in prior-art optical systems of the natures above described is that the beam splitter prism incorporated therein has a bulky parallelepiped construction and for this reason requires a large space for the accommodation of the prism in the signal pickup device using the optical system. Another drawback is that the pentahedral component segments constituting the beam splitter prism have a total of ten surfaces to be polished and require a large number of steps and meticuous and highly skilled control over the accuracy of geometrical dimensions of the prism during fabrication of the prism. A third drawback inherent in prior-art optical systems of the described natures consists in the difficulty in taking precise alignment between the parallelepiped beam splitter prism and the lens or each of the lenses provided in combination with the prism.
The present invention contemplates elimination of these and other drawbacks inherent in prior-art optical systems of the described natures.
It is, accordingly, an important object of the present invention to provide an improved optical system featuring, inter alia, a beam splitter prism having a simple and compact construction that will permit of significant reduction of the overall dimensions of a signal pickup device into which the optical system may be incorporated.
It is another important object of the present invention to provide an improved optical system featuring a beam splitter prism composed of component segments each having a significantly reduced number of surfaces to be polished during fabrication of the prism.
It is still another important object of the present invention to provide an improved optical system in which alignment can be easily and accurately taken between optical elements including a beam splitter prism.
It is, yet, another important object of the present invention to provide an optical system which is simple and compact in construction and which is easy and economical to manufacture.